linux-kernel - Re: FPU register granularity [Was: Re: [PATCH crypto-stable] crypto: arch/lib

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Message-ID: <CAHmME9pRFGRi8oxazFrd05S+m=_s7=WF5x_jfUAE_Qt+c5-anA@mail.gmail.com>
Date:   Mon, 20 Apr 2020 22:25:33 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ebiggers@...gle.com" <ebiggers@...gle.com>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: FPU register granularity [Was: Re: [PATCH crypto-stable] crypto:
 arch/lib - limit simd usage to PAGE_SIZE chunks]

On Mon, Apr 20, 2020 at 10:14 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Hi David,
>
> On Mon, Apr 20, 2020 at 2:32 AM David Laight <David.Laight@...lab.com> wrote:
> > Maybe kernel_fp_begin() should be passed the address of somewhere
> > the address of an fpu save area buffer can be written to.
> > Then the pre-emption code can allocate the buffer and save the
> > state into it.
>
> Interesting idea. It looks like `struct xregs_state` is only 576
> bytes. That's not exactly small, but it's not insanely huge either,
> and maybe we could justifiably stick that on the stack, or even
> reserve part of the stack allocation for that that the function would
> know about, without needing to specify any address.

Hah-hah, nevermind here. extended_state_area is of course huge,
bringing the whole structure to a whopping 3k with avx512. :)